This invention relates to multilayered tapes that have a pressure-sensitive adhesive surface and a separate tack-free adhesion promoting or priming surface. More particularly it relates to conformable, multilayered tapes that are employed to adhere fusible metal alloy to ophthalmic lens blanks. The metal alloy bonds a lens block to the ophthalmic lens blank for use during surfacing (i.e., grinding, fining and polishing) and edging operations.
The term "pressure-sensitive adhesive", as it is used herein, means a compound or composition that is dry (solvent free), aggressively and permanently tacky at room temperature, and firmly adheres to a substrate upon mere contact without the need of application of more than finger or hand pressure. The term "priming surface" means a surface that contains a material that improves or increases the bond between two surfaces.
In making finished ophthalmic lenses, particularly presciption lenses for eyeglasses, it is customary to begin with semi-finished lens blanks made from glass or plastic. The blanks have a finished, polished front surface and an unfinished back surface. They are surfaced to a particular prescription by grinding material from the unfinished back surface followed by fining and polishing so that they acquire the optical refractive properties specified in the prescription. The lenses may then be shaped or edged to fit the spectacle frame selected by the wearer.
It is essential that a lens be positioned accurately and held securely during the surfacing and edging operations. However, edge clamping techniques, such as mounting the lens in a vise or in the jaws of a chuck, are unsuitable for holding the lens because material is removed from both its back surface and edges. Thus, it is necessary that the lens be held by an adhesive means which secures it by the finished surface in the appropriate position in the grinding machine. This may be accomplished by "blocking" the lens, e.g., adhering a lens block to the lens by means of a fusible metal alloy.
The alloy is applied in a molten state and subsequently allowed to harden to form a rigid layer of predetermined size and shape that bonds the lens to the lens block. While the alloy conforms well to the front surface of the lens blank, the strength of the bond between the alloy and the lens blank is low. Consequently, primers are needed to obtain adequate bonding between alloy and lens. Primers, however, cause certain problems. For example, they are typically applied from a solution by, for example, brushing or spraying. Consequently the solvent must be allowed to evaporate before the surfacing and edging processes can proceed. This causes inconvenience and delay in processing the lens blanks.
Other techniques of blocking lens blanks have also been tried. Thus, sticky substances, such as pitch or wax, double-sided sticky constructions such as pads or foams coated on each face with adhesive, and epoxy adhesives have been used. These means also have not proven entirely satisfactory. For example, the residue left by pitch, wax, and adhesives require extensive clean-up of both lens and block. This causes delay and added expense in the processing operations. Additionally these techniques provide less rigid mounting means than do the alloy bonded blocks. Consequently, it is more difficult to assure that the lens will be properly positioned throughout the entire surfacing and edging processes.
Additionally, pads or foams are typically opaque so that it is difficult to properly align the lens in the surfacing or edging apparatus. Additionally, the pads and foams are not satisfactorily conformable to the complex curvature of a lens face. Thus wrinkles, folds, air bubbles and other discontinuities between the lens blank and the pad or form are present when they are employed.
The present invention, however, overcomes these disadvantages. It provides a multilayered tape construction which firmly bonds the fusible metal alloy to the lens blanks. The tapes are conformable, that is, they follow the curvature of the lens blanks without any wrinkles or air bubbles. Moreover the tapes are preferably translucent, that is, they permit light to pass therethrough. As a result, the lens may be visually aligned in the appropriate device prior to blocking. Still further, when tapes of the present invention are removed from the lens they leave virtually no adhesive residue. Thus, messy and time consuming cleaning operations need not be performed on the lens before it can be used. Additionally, the preferred tapes of the invention do not leave any residue on the metal alloy when removed therefrom. Thus, no cleaning is required on the alloy before it can be recycled.
Despite this clean removability, the tapes of the present invention exhibit excellent adhesion to both the lens blank and the alloy. Additionally, the tapes of the invention are able to withstand the shear forces encountered during the surfacing and edging operations. As a result, lenses are held in accurate position throughout these operations.
An added benefit offered by the tape of the present invention is the protection provided to the lenses from thermal and mechanical shock. Thermal protection is particularly important because, in the case of plastic lenses, it is possible for heat distortion to occur in the lens blank when the molten fusible metal alloy makes contact with it. In the completed lens, this distortion will cause abberations from the desired prescription in those areas where it occurred. However, when tapes of the present invention are employed, they offer a significant degree of protection from such distortion.